The present invention relates to a process for the catalytic gas-phase oxidation of propene to acrylic acid, in which a reaction gas starting mixture which comprises propene, molecular oxygen and at least one inert gas and contains the molecular oxygen and the propene in a molar O2: C3H6 ratio of xe2x89xa71 is passed, in a first reaction stage, using a propene loading of xe2x89xa7160 l (S.T.P.) of propene/l of fixed catalyst bed 1 per h, over a fixed catalyst bed 1 which is arranged in two spatially successive reaction zones A, B, the temperature of the reaction zone A being from 300 to 370xc2x0 C. and the temperature of the reaction zone B being from 305 to 380xc2x0 C. and simultaneously at least 5xc2x0 C. above the temperature of the reaction zone A and the active material of which being at least one multimetal oxide containing at least the elements Mo, Fe and Bi, in such a way that the reaction zone A extends to a propene conversion of from 40 to 80 mol % and the propene conversion during a single pass through the fixed catalyst bed 1 is xe2x89xa790 mol % and the associated selectivity of the acrolein formation and of the acrylic acid byproduct formation together are xe2x89xa790 mol %, and the resulting product gas mixture, which contains the molecular oxygen and the acrolein in a molar O2: C3H4O ratio of xe2x89xa70.5, is passed, in a second reaction stage, over a fixed catalyst bed 2 which is arranged either in a single reaction zone C or in two spatially successive reaction zones D, E, the temperature of the reaction zone C being from 230 to 300xc2x0 C. and the temperature of the reaction zone D being from 230 to 280xc2x0 C. and the temperature of the reaction zone E being from 250 to 300xc2x0 C. and simultaneously at least 5xc2x0 C. above the temperature of the reaction zone D and the active material of which being at least one multimetal oxide containing at least the elements Mo and V, in such a way that the acrolein conversion during a single pass through the reaction zone C or the reaction zones D and E is xe2x89xa790 mol % and the selectivity of the acrylic acid formation balanced over all reaction zones and based on propene converted is xe2x89xa780 mol %, the sequence in which the reaction gas starting mixture flows through the reaction zones corresponding to the alphabetic sequence of the reaction zones.
Acrylic acid is a key monomer which is used as such or in the form of its alkyl esters for producing, for example, polymers suitable as adhesives.
The preparation of acrylic acid can be carried out, for example, by two-stage catalytic gas-phase oxidation of propene to acrylic acid.
The process for the two-stage catalytic gas-phase oxidation of propene to acrylic acid is generally known (cf. for example DE-A 3002829). In particular the two reaction stages are known per se (cf. for example EP-A 714700, EP-A 700893, EP-A 15565, DE-C 2830765, DE-C 3338380, JP-A 91/294239, EP-A 807465, WO 98/24746, EP-B 279374, DE-C 2513405, DE-A 3300044, EP-A 575897 and DE-A 19855913).
In particular, it has been proposed to realize the two reaction stages in two tube-bundle reactors which have a plurality of catalyst tubes and each of which has two temperature zones (cf. for example DE-A 19948241, DE-A 19948523, DE-A 19910506, DE-A 19910508 and DE-A 19948248), which is considered advantageous particularly in the case of high loading with starting materials.
In all abovementioned cases, it is advisable to use, for each reaction stage, one tube-bundle reactor comprising a plurality of catalyst tubes, i.e. to carry out the process as a whole in two tube-bundle reactors spatially separated from one another and arranged one behind the other.
The background of this recommendation is the fact that, for example, U.S. Pat. No. 4,029,636 discloses that MoO3 volatilizes from the multimetal oxide catalyst of the first reaction stage operated at the higher temperature and is partially deposited again in the fixed catalyst bed 2 of the second reaction stage having the lower operating temperature.
Consequently, an increase in pressure drop of the reaction gas mixture flowing through the two fixed catalyst beds occurs over time (fixed catalyst bed 2 slowly becomes blocked).
According to EP-A 614 872, the pressure drop established over time is even further increased by the fact that organic materials, for example solid carbon, are additionally regularly deposited in the fixed catalyst bed 2.
According to U.S. Pat. No. 4,029,639, the problem described can be remedied when the two reaction stages are realized in two spatially separated tube-bundle reactors arranged one behind the other and the major part of the abovementioned deposit is effected in an intermediate condenser which is mounted between the two tube-bundle reactors and is equipped with inert solids.
For example, DE-A 2830765 and EP-A 911313 recommend carrying out two-stage catalytic gas-phase oxidation of propene to acrylic acid in a single tube-bundle reactor. However, the propene loading of the fixed catalyst bed 1 in all exemplary embodiments is xe2x89xa7100 l (S.T.P.) of propene/l of fixed catalyst bed 1 per h. Lower propene loadings are however equivalent to lower MoO3 volatilization since, for example, the amount of water of reaction formed within a specific period (assuming comparable conversions) at lower loading is smaller than at high loading. However, U.S. Pat. No. 4,029,636 discloses that the MoO3 volatilization is promoted in particular by steam.
On the other hand, at a higher propene loading per hour, a larger amount of MoO3 can volatilize since of course a larger saturation amount of MoO3 corresponds to a larger amount of gas.
On the other hand, realizing the two-stage catalytic gas-phase fixed-bed oxidation of propene for the preparation of acrylic acid in two spatially separated tube-bundle reactors comprising a plurality of catalyst tubes entails particularly high capital costs.
It is an object of the present invention to provide a process for the two-stage catalytic gas-phase fixed-bed oxidation of propene to acrylic acid in a single tube-bundle reactor comprising a plurality of catalyst tubes, and to do so with a high propene loading of the fixed catalyst bed 1, which process has the advantage of a pressure drop rapidly increasing over time along the catalyst tubes only in reduced form.
We have found, surprisingly, that this object is achieved if the first reaction stage is realized, as described at the outset, in two spatially successive temperature zones A, B.
The present invention therefore relates to a process for the catalytic gas-phase oxidation of propene to acrylic acid, in which a reaction gas starting mixture which comprises propene, molecular oxygen and at least one inert gas and contains the molecular oxygen and the propene in a molar O2: C3H6 ratio of xe2x89xa71 is passed, in a first reaction stage, using a propene loading of xe2x89xa7160 l (S.T.P.) of propene/l of fixed catalyst bed 1 per h, over a fixed catalyst bed 1 which is arranged in two spatially successive reaction zones A, B, the temperature of the reaction zone A being from 300 to 370xc2x0 C. and the temperature of the reaction zone B being from 305 to 380xc2x0 C. and simultaneously at least 5xc2x0 C. above the temperature of the reaction zone A and the active material of which being at least one multimetal oxide containing at least the elements Mo, Fe and Bi, in such a way that the reaction zone A extends to a propene conversion of from 40 to 80 mol % and the propene conversion during a single pass through the fixed catalyst bed 1 is xe2x89xa790 mol % and the associated selectivity of the acrolein formation and of the acrylic acid byproduct formation (overall selectivity of formation of desired product) together are xe2x89xa790 mol %, and the resulting product gas mixture, which contains the molecular oxygen and the acrolein in a molar O2: C3H4O ratio of xe2x89xa70.5, is passed, in a second reaction stage, over a fixed catalyst bed 2 which is arranged either in a single reaction zone C or in two spatially successive reaction zones D, E, the temperature of the reaction zone C being from 230 to 300xc2x0 C. and the temperature of the reaction zone D being from 230 to 280xc2x0 C. and the temperature of the reaction zone E being from 250 to 300xc2x0 C. and simultaneously at least 5xc2x0 C. above the temperature of the reaction zone D and the active material of which being at least one multimetal oxide containing at least the elements Mo and V, in such a way that the acrolein conversion during a single pass through the reaction zone C or the reaction zones D and E is xe2x89xa790 mol % and the selectivity of the acrylic acid formation balanced over all reaction zones and based on propene converted is xe2x89xa780 mol %, the sequence in which the reaction gas starting mixture flows through the reaction zones corresponding to the alphabetic sequence of the reaction zones, wherein both reaction stages, i.e. both the fixed catalyst beds 1 and 2 and the reaction zones A, B and C or A, B, D and E, are present in a single tube-bundle reactor comprising a plurality of catalyst tubes.
The temperature of a reaction zone is understood here as meaning the temperature of the fixed catalyst bed fraction present in the reaction zone when the novel process is carried out in the absence of a chemical reaction.
The loading of a catalyst bed with reactant is understood here very generally as meaning the amount of reactant in liters under standard conditions (=l (S.T.P.); the volume in liters which the corresponding amount of reactant would occupy under standard temperature and pressure conditions, i.e. at 25xc2x0 C. and 1 bar) which is passed as a component of the reaction gas starting mixture per hour through one liter of catalyst bed.
It is preferable according to the invention if the reaction zone A extends to a propene conversion of from 50 to 70, particularly preferably from 65 to 75, mol %.
According to the invention, the temperature of the reaction zone B is advantageously from 310 to 370xc2x0 C., particularly advantageously from 320 to 370xc2x0 C.
Furthermore, the temperature of the reaction zone B is preferably at least 10xc2x0 C. above the temperature of the reaction zone A.
The higher the chosen propene loading of the fixed catalyst bed 1 in the novel process, the greater should be the chosen difference between the temperature of the reaction zone A and the temperature of the reaction zone B. Usually, however, the abovementioned temperature difference in the novel process is not more than 50xc2x0 C., i.e. the difference between the temperature of the reaction zone A and the temperature of the reaction zone B may be, according to the invention, up to 20xc2x0 C., up to 25xc2x0 C., up to 30xc2x0 C., up to 40xc2x0 C., up to 45xc2x0 C. or up to 50xc2x0 C.
As a rule, the propene conversion in the novel process is xe2x89xa792 mol % or xe2x89xa794 mol %, based on a single pass through the first reaction stage. With suitable catalyst choice in a manner known per se, the selectivity of formation of the desired product is usually xe2x89xa792 mol % or xe2x89xa794 mol %, frequently xe2x89xa795 mol % or xe2x89xa796 mol % or xe2x89xa797 mol %.
Surprisingly, the abovementioned applies according to the invention not only in the case of propene loadings of the fixed catalyst bed 1 of xe2x89xa7165 l (S.T.P.)/lxc2x7h or of xe2x89xa7170 l (S.T.P.)/lxc2x7h or xe2x89xa7175 l (S.T.P.)/lxc2x7h or xe2x89xa7180 l (S.T.P.)/lxc2x7h, but also in the case of propene loadings of the fixed catalyst bed 1 of xe2x89xa7185 l (S.T.P.)/lxc2x7h or xe2x89xa7190 l (S.T.P.)/lxc2x7h or xe2x89xa7200 l (S.T.P.)/lxc2x7h or xe2x89xa7210 l (S.T.P.)/lxc2x7h and in the case of loadings xe2x89xa7220 l (S.T.P.)/lxc2x7h or xe2x89xa7230 l (S.T.P.)/lxc2x7h, or xe2x89xa7240 l (S.T.P.)/lxc2x7h or xe2x89xa7250 l (S.T.P.)/lxc2x7h.
It is surprising that the abovementioned values can be reached even when the inert gas used according to the invention comprises xe2x89xa730% by volume or xe2x89xa740% by volume or xe2x89xa7250% by volume or xe2x89xa760% by volume or xe2x89xa770% by volume or xe2x89xa780% by volume or xe2x89xa7290% by volume or xe2x89xa795% by volume of molecular nitrogen. In the case of propene loadings above 250 l (S.T.P.)/lxc2x7h, the presence of inert (inert diluent gases should in general be those which undergo less than 5%, preferably less than 2%, conversion during a single pass) diluent gases, such as propane, ethane, methane, pentane, butane, CO2, CO, steam and/or noble gases is recommended for the novel process. Of course, these gases and their mixtures can however be present even at lower loadings or can be used as sole diluent gases. It is furthermore surprising that the novel process can be carried out using a fixed catalyst bed 1 which, considered over the reaction zones A, B, is homogeneous, i.e. chemically uniform, without significantly suffering from reduced conversion and/or reduced selectivity.
Usually, the propene loading of the fixed catalyst bed 1 in the novel process will not exceed 600 l (S.T.P.)/lxc2x7h. Typically, the propene loadings of the fixed catalyst bed 1 in the novel process are xe2x89xa6300 l (S.T.P.)/lxc2x7h, frequently xe2x89xa6250 l (S.T.P.)/lxc2x7h, without significant reduction in conversion and selectivity.
In the novel process, the operating pressure may be either below atmospheric pressure (e.g. down to 0.5 bar) or above atmospheric pressure. Typically, the operating pressure is from 1 to 5, frequently from 1.5 to 3.5, bar. Usually, the reaction pressure does not exceed 100 bar.
According to the invention, the molar O2:C3H6 ratio in the reaction gas starting mixture must be xe2x89xa71. Usually, this ratio is xe2x89xa63. According to the invention, the molar O2:C3E6 ratio in the reaction starting mixture is frequently xe2x89xa71.5 and xe2x89xa62.0.
A suitable source of the molecular oxygen required in the novel process is either air or air depleted with respect to molecular nitrogen (e.g. xe2x89xa790% by volume of O2, xe2x89xa610% by volume of N2).
According to the invention, the propene fraction in the reaction gas starting mixture may be, for example, from 4 to 15, frequently from 5 to 12, % by volume or from 5 to 8% by volume (based in each case on the total volume).
Frequently, the novel process is carried out at a volume ratio of propene to oxygen to inert gases (including steam) in the reaction gas starting mixture of 1:(1.0 to 3.0):(5 to 25), preferably 1:(1.7 to 2.3):(10 to 15).
Usually, the reaction gas starting mixture contains essentially no further components apart from said constituents.
Suitable fixed-bed catalysts 1 for the novel process are all those whose active material is at least one Moxe2x80x94, Bixe2x80x94 and Fe-containing multimetal oxide.
In other words, it is possible in principle to use, according to the invention, all those catalysts which are disclosed in DE-C 3338380, DE-A 19902562, EP-A 15565, DE-C 2830765, EP-A 807465, EP-A 279374, DE-A 3300044, EP-A 575897, U.S. Pat. No. 4,438,217, DE-A 19855913, WO 98/24746, DE-A 19746210 (those of the formula II), JP-A 91/294239, EP-A 293224 and EP-A 700714. This applies in particular to the exemplary embodiments in these publications, of which those of EP-A 15565, EP-A 575897, DE-A 19746210 and DE-A 19855913 are particularly preferred. Particularly noteworthy in this context are a catalyst according to Example 1c of EP-A 15565 and a catalyst that has to be prepared in a corresponding manner but whose active material has the composition Mo12Ni6.5Zn2Fe2Bi1P0.0065K0.06Ox.10SiO2. Also noteworthy are the example with the consecutive number 3 in DE-A 19855913 (stoichiometry: Mo12Co7Fe3Bi0.6K0.08Si1.6Ox) as unsupported catalyst in the form of a hollow cylinder measuring 5 mmxc3x973 mmxc3x972 mm (external diameterxc3x97heightxc3x97internal diameter) and the unsupported catalyst comprising multimetal oxide II and according to Example 1 of DE-A 19746210. The multimetal oxide catalysts of U.S. Pat. No. 4,438,217 may furthermore be mentioned. The latter applies in particular when these hollow cylinders measure 5 mmxc3x972 mmxc3x972 mm, or 5 mmxc3x973 mmxc3x972 mm, or 6 mmxc3x973 mmxc3x973 mm, or 7 mmxc3x973 mmxc3x974 mm (in each case external diameterxc3x97heightxc3x97internal diameter).
A large number of the multimetal oxide active materials suitable according to the invention for the fixed catalyst bed 1 can be subsumed under the formula I
Mo12BiaFebXc1Xd2Xe3Xf4Onxe2x80x83xe2x80x83(I),
where
X1 is nickel and/or cobalt,
X2 is thallium, an alkali metal and/or an alkaline earth metal,
X3 is zinc, phosphorus, arsenic, boron, antimony, tin, cerium, lead and/or tungsten,
X4 is silicon, aluminum, titanium and/or zirconium,
a is from 0.5 to 5,
b is from 0.01 to 5, preferably from 2 to 4,
c is from 0 to 10, preferably from 3 to 10,
d is from 0 to 2, preferably from 0.02 to 2,
e is from 0 to 8, preferably from 0 to 5,
f is from 0 to 10 and
n is a number which is determined by the valency and frequency of the elements other than oxygen in I.
They are obtainable in a manner known per se (cf. for example DE-A 4023239) and are usually shaped as such in spheres, rings or cylinders or used in the form of coated catalysts, i.e. premolded, inert supports coated with active material. However, they can of course also be used in powder form as catalysts. According to the invention, the Bixe2x80x94, Moxe2x80x94 and Fe-comprising multimetal oxide catalyst ACS-4 from Nippon Shokubai can of course also be used.
In principle, the active materials suitable according to the invention for the fixed catalyst bed 1, in particular those of the formula I, can be prepared in a simple manner by producing, from suitable sources of their elemental constituents, a very intimate, preferably finely divided dry blend having a composition corresponding to their stoichiometry and calcining said dry blend at from 350 to 650xc2x0 C. The calcination can be carried out both under inert gas and under an oxidizing atmosphere, e.g. air (mixture of inert gas and oxygen) and also under a reducing atmosphere (e.g. mixture of inert gas, NH3, CO and/or H2). The duration of calcination may be from a few minutes to a few hours and usually decreases with increasing temperature. Suitable sources of the elemental constituents of the multimetal oxide active materials I are those compounds which are already oxides and/or those compounds which can be converted into oxides by heating, at least in the presence of oxygen.
In addition to the oxides, suitable such starting compounds are in particular halides, nitrates, formates, oxalates, citrates, acetates, carbonates, amine complexes, ammonium salts and/or hydroxides (compounds such as NH4OH, (NH4)2CO3, NH4NO3, NH4CHO2, CH3COOH, NH4CH3CO2 and/or ammonium oxalate, which are decomposed and/or can be decomposed at the latest during the subsequent calcination to give compounds escaping in gaseous form, may additionally be incorporated into the intimate dry blend).
The thorough mixing of the starting compounds for the preparation of multimetal oxide materials I can be carried out in dry or in wet form. If it is effected in dry form, the starting compounds are expediently used in the form of finely divided powders and, after mixing and any compaction, are subjected to the calcination. However, the thorough mixing is preferably effected in wet form. Usually, the starting compounds are mixed with one another in the form of an aqueous solution and/or suspension. Particularly intimate dry blends are obtained in the mixing method described when exclusively sources of the elemental constituents in dissolved form are used as starting materials. The preferably used solvent is water. The aqueous material obtained is then dried, the drying process preferably being carried out by spray-drying the aqueous mixture with outlet temperatures of from 100 to 150xc2x0 C.
The multimetal oxide materials suitable according to the invention as active material for the fixed-bed catalysts 1, in particular those of the formula I, can be used for the novel process both in powder form and after shaping to give specific catalyst geometries, it being possible to effect the shaping before or after the final calcination. For example, unsupported catalysts can be prepared from the powder form of the active material or its uncalcined and/or partially calcined precursor material by compaction to give the desired catalyst geometry (for example by pelleting or extrusion), it being possible, if required, to add assistants, e.g. graphite or stearic acid as lubricants and/or molding assistants and reinforcing agents, such as microfibers of glass, asbestos, silicon carbide or potassium titanate. Suitable geometries for unsupported catalysts are, for example, solid cylinders or hollow cylinders having an external diameter and a length of from 2 to 10 mm. In the case of the hollow cylinders, a wall thickness of 1 to 3 mm is expedient. Of course, the unsupported catalyst can also have spherical geometry, where the sphere diameter can be from 2 to 10 mm.
The shaping of the pulverulent active material or its pulverulent, still uncalcined and/or partially calcined precursor material can of course also be effected by application to premolded inert catalyst supports. The coating of the supports for the preparation of the coated catalysts is carried out as a rule in a suitable rotatable container, as disclosed, for example, in DE-A 2909671, EP-A 293859 or EP-A 714700. Expediently, for coating the supports, the powder material to be applied is moistened and, after application, is dried again, for example by means of hot air. The coat thickness of the powder material applied to the support is expediently chosen to be in the range from 10 to 1000 xcexcm, preferably from 50 to 500 xcexcm, particularly preferably from 150 to 250 xcexcm.
Conventional porous or nonporous aluminas, silica, thorium dioxide, zirconium dioxide, silicon carbide or silicates, such as magnesium silicate or aluminum silicate can be used as support materials. The supports may have a regular or irregular shape, those having a regular shape and substantial surface roughness, e.g. spheres or hollow cylinders, being preferred. The use of essentially nonporous, spherical steatite supports which have a rough surface and whose diameter is from 1 to 8 mm, preferably from 4 to 5 mm, is suitable. However, the use of cylinders whose length is from 2 to 10 mm and whose external diameter is from 4 to 10 mm as supports is also suitable. When rings suitable according to the invention are used as supports, the wall thickness is moreover usually from 1 to 4 mm. Annular supports preferably to be used according to the invention have a length of from 2 to 6 mm, an external diameter of from 4 to 8 mm and a wall thickness of from 1 to 2 mm. Rings measuring 7 mmxc3x973 mmxc3x974 mm (external diameterxc3x97lengthxc3x97internal diameter) are also particularly suitable according to the invention as supports. The fineness of the catalytically active oxide materials to be applied to the surface of the support is of course adapted to the desired coat thickness (cf. EP-A 714 700).
Advantageous multimetal oxide active materials to be used according to the invention for the fixed catalyst bed 1 are furthermore materials of the formula II
[Y1axe2x80x2Y2bxe2x80x2Oxxe2x80x2]p[Y3cxe2x80x2Y4dxe2x80x2Y5exe2x80x2Y6fxe2x80x2Y7gxe2x80x2Y2hxe2x80x2Oyxe2x80x2]qxe2x80x83xe2x80x83(II),
where
Y1 is bismuth, tellurium, antimony, tin and/or copper,
Y2 is molybdenum and/or tungsten,
Y3 is an alkali metal, thallium and/or samarium,
Y4 is an alkaline earth metal, nickel, cobalt, copper, manganese, zinc, tin, cadmium and/or mercury,
Y5 is iron, chromium, cerium and/or vanadium,
Y6 is phosphorus, arsenic, boron and/or antimony,
Y7 is a rare earth metal, titanium, zirconium, niobium, tantallum, rhenium, ruthenium, rhodium, silver, gold, aluminum, gallium, indium, silicon, germanium, lead, thorium and/or uranium,
axe2x80x2 is from 0.01 to 8,
bxe2x80x2 is from 0.1 to 30,
cxe2x80x2 is from 0 to 4,
dxe2x80x2 is from 0 to 20,
exe2x80x2 is from 0 to 20,
fxe2x80x2 is from 0 to 6,
gxe2x80x2 is from 0 to 15,
hxe2x80x2 is from 8 to 16,
xxe2x80x2 and yxe2x80x2 are numbers which are determined by the valency and frequency of the elements other than oxygen in II, and
p and q are numbers whose ratio p/q is from 0.1 to 10,
containing three-dimensional regions of the chemical composition Y1axe2x80x2Y2bxe2x80x2Oxxe2x80x2, which are delimited from their local environment owing to their composition differing from their local environment and whose largest diameter (longest line passing through the center of gravity of the region and connecting two points present on the surface (interface) of the region) is from 1 nm to 100 xcexcm, frequently from 10 nm to 500 nm or from 1 xcexcm to 50 or 25 xcexcm.
Particularly advantageous novel multimetal oxide materials II are those in which Y1 is bismuth.
Preferred among these in turn are those of the formula III
[Biaxe2x80x3Z2bxe2x80x3Oxxe2x80x3]pxe2x80x3[Z212Z3cxe2x80x3Z4dxe2x80x3Feexe2x80x3Z5fxe2x80x3Z6gxe2x80x3Z7hxe2x80x3Oyxe2x80x3]qxe2x80x3xe2x80x83xe2x80x83(III)
where
Z2 is molybdenum and/or tungsten,
Z3 is nickel and/or cobalt,
Z4 is thallium, an alkali metal and/or an alkaline earth metal,
Z5 is phosphorus, arsenic, boron, antimony, tin, cerium and/or lead,
Z6 is silicon, aluminum, titanium and/or zirconium,
Z7 is copper, silver and/or gold,
axe2x80x3 is from 0.1 to 1,
bxe2x80x3 is from 0.2 to 2,
cxe2x80x3 is from 3 to 10,
dxe2x80x3 is from 0.02 to 2,
exe2x80x3 is from 0.01 to 5, preferably from 0.1 to 3,
fxe2x80x3 is from 0 to 5,
gxe2x80x3 is from 0 to 10,
hxe2x80x3 is from 0 to 1,
xxe2x80x3 and yxe2x80x3 are numbers which are determined by the valency and frequency of the elements other than oxygen in III, and
pxe2x80x3 and qxe2x80x3 are numbers whose ratio pxe2x80x3/qxe2x80x3 is from 0.1 to 5, preferably from 0.5 to 2,
those materials III in which Z2bxe2x80x3 is (tungsten)bxe2x80x3 and Z212 is (molybdenum)12 being very particularly preferred.
It is also advantageous if at least 25 mol % (preferably at least 50, particularly preferably at least 100, mol %) of the total amount [Y1axe2x80x2Y2bxe2x80x2Oxxe2x80x2]p([Biaxe2x80x3Z2bxe2x80x3Oxxe2x80x3]pxe2x80x3) of the multimetal oxide materials II (multimetal oxide materials III) suitable according to the invention are present in the multimetal oxide materials II (multimetal oxide materials III) suitable according to the invention in the form of three-dimensional regions of the chemical composition Y1axe2x80x2Y2bxe2x80x2Oxxe2x80x2[Biaxe2x80x3Z2bxe2x80x3Oxxe2x80x3] which are delimited from their local environment owing to their chemical composition differing from their local environment and whose largest diameter is from 1 nm to 100 xcexcm.
Regarding the shaping, the statements made in the case of the catalysts comprising multimetal oxide materials I are applicable with respect to catalysts comprising the multimetal oxide materials II.
In contrast to the fixed catalyst bed 1, according to the invention the fixed catalyst bed 2 may be present either in one reaction zone having a uniform temperature (reaction zone C) or in two spatially successive reaction zones D, E having temperatures differing from one another.
It is expedient according to the invention if the temperature of the reaction zone C is from 230 to 285xc2x0 C. Furthermore according to the invention, the temperature of the reaction zone D is expediently from 245 to 275xc2x0 C. and, according to the invention, the temperature of the reaction zone E is advantageously at least 20xc2x0 C. above the temperature of the reaction zone D and is advantageously from 265 to 295xc2x0 C.
The higher the chosen propene loading of the fixed catalyst bed 1, and hence entirely automatically the acrolein loading of the fixed catalyst bed 2, in the novel process, the more advantageous is the use of two temperature zones D, E instead of one temperature zone C. The difference between the temperature of the reaction zone D and the temperature of the reaction zone E should be chosen to be the greater the higher the chosen reactant loading.
However, the abovementioned temperature difference in the novel process is usually not more than 40xc2x0 C., i.e. the difference between the temperature of the reaction zone D and the temperature of the reaction zone E may be, according to the invention, up to 15xc2x0 C., up to 25xc2x0 C., up to 30xc2x0 C., up to 35xc2x0 C. or up to 40xc2x0 C.
It is important according to the invention that the oxygen required in the second reaction stage is already present in the reaction gas starting mixture for the first reaction stage.
Apart from the constituents stated in this publication, the reaction gas starting mixture usually contains essentially no further components.
The acrolein loading of the fixed catalyst bed 2 in the novel process is an automatic consequence of the propene loading chosen in the first reaction stage for the fixed catalyst bed 1 and the reaction conditions chosen in the first reaction stage, including the chosen composition of the reaction gas starting mixture.
In the novel process, the acrolein loading of the fixed catalyst bed 2 is as a rule about 10 l (S.T.P.)/lxc2x7h, frequently about 20 or 25 l (S.T.P.)/lxc2x7h, below the propene loading of the fixed catalyst bed 1. This is primarily due to the fact that, in the first reaction stage, both conversion and selectivity with respect to acrolein generally do not reach 100%.
When the fixed catalyst bed 2 is chosen in a suitable manner known per se, in the novel process the selectivity of the acrylic acid formation, balanced over both reaction stages and based on propene converted, may be xe2x89xa783 mol %, frequently xe2x89xa785 mol % or xe2x89xa788 mol %, often xe2x89xa790 mol % or xe2x89xa793 mol %, even in the case of very high propene loadings of fixed catalyst bed 1.
Suitable fixed-bed catalysts 2 to be used according to the invention in the gas-phase catalytic acrolein oxidation in the second reaction stage are all those whose active material is at least one Moxe2x80x94 and V-containing multimetal oxide.
Suitable multimetal oxide active materials of this type are described, for example, in U.S. Pat. No. 3,775,474, U.S. Pat. No. 3,954,855, U.S. Pat. No. 3,893,951 and U.S. Pat. No. 4,339,355. Also particularly suitable are the multimetal oxide active materials of EP-A 427 508, of DE-A 2 909 671, of DE-C 31 51 805, of German Published Application DAS 2 626 887, of DE-A 43 02 991, of EP-A 700 893, of EP-A 714 700 and of DE-A 19 73 6105. Particularly preferred in this context are the exemplary embodiments of EP-A 714 700 and of DE-A 19 73 6105.
A large number of the multimetal oxide active materials suitable for fixed-bed catalysts 2 can be subsumed under the formula IV
Mo12VaX1bX2cX3dX4eX5fX6gOnxe2x80x83xe2x80x83(IV),
where
X1 is W, Nb, Ta, Cr and/or Ce,
X2 is Cu, Ni, Co, Fe, Mn and/or Zn,
X3 is Sb and/or Bi,
X4 is one or more alkali metals,
X5 is one or more alkaline earth metals,
X6 is Si, Al, Ti and/or Zr,
a is from 1 to 6,
b is from 0.2 to 4,
c is from 0.5 to 18,
d is from 0 to 40,
e is from 0 to 2,
f is from 0 to 4,
g is from 0 to 40 and
n is a number which is determined by the valency and frequency of the elements other than oxygen in IV.
Preferred embodiments within the active multimetal oxides IV are those which are covered by the following meanings of the variables of the formula IV:
X1 is W, Nb, and/or Cr,
X2 is Cu, Ni, Co, and/or Fe,
X3 is Sb,
X4 is Na and/or K,
X5 is Ca, Sr and/or Ba,
X6 is Si, Al, and/or Ti,
a is from 1.5 to 5,
b is from 0.5 to 2,
c is from 0.5 to 3,
d is from 0 to 2,
e is from 0 to 0.2,
f is from 0 to 1 and
n is a number which is determined by the valency and frequency of the elements other than oxygen in IV.
However, very particularly preferred multimetal oxides IV are those of the formula V
Mo12Vaxe2x80x2Y1bxe2x80x2Y2cxe2x80x2Y5fxe2x80x2Y6gxe2x80x2Onxe2x80x2xe2x80x83xe2x80x83(V)
where
Y1 is W and/or Nb,
Y2 is Cu and/or Ni,
Y5 is Ca and/or Sr,
Y6 is Si and/or Al,
axe2x80x2 is from 2 to 4,
bxe2x80x2 is from 1 to 1.5,
cxe2x80x2 is from 1 to 3,
fxe2x80x2 is from 0 to 0.5,
gxe2x80x2 is from 0 to 8 and
nxe2x80x2 is a number which is determined by the valency and frequency of the elements other than oxygen in V.
The multimetal oxide active materials (IV) suitable according to the invention are obtainable in a manner known per se, for example disclosed in DE-A 4335973 or in EP-A 714700.
In principle, multimetal oxide active materials suitable according to the invention for fixed-bed catalysts 2, in particular those of the formula IV, can be prepared in a simple manner by producing, from suitable sources of their elemental constituents, a very intimate, preferably finely divided dry blend having a composition corresponding to their stoichiometry and calcining said dry blend at from 350 to 600xc2x0 C. The calcination can be carried out both under inert gas and under an oxidizing atmosphere, e.g. air (mixture of inert gas and oxygen) and also under a reducing atmosphere (e.g. mixtures of inert gas and reducing gases, such as H2, NH3, CO, methane and/or acrolein or said reducing gases by themselves). The duration of calcination may be from a few minutes to a few hours and usually decreases with increasing temperature. Suitable sources of the elemental constituents of the multimetal oxide active materials IV are those compounds which are already oxides and/or those compounds which can be converted into oxides by heating, at least in the presence of oxygen.
The thorough mixing of the starting compounds for the preparation of multimetal oxide materials IV can be carried out in dry or in wet form. If it is effected in dry form, the starting compounds are expediently used in the form of finely divided powders and, after the mixing and any compaction, are subjected to the calcination. However, thorough mixing is preferably effected in wet form.
Usually, the starting compounds are mixed with one another in the form of an aqueous solution and/or suspension. Particularly intimate dry blends are obtained in the mixing method described when exclusively dissolved sources of the elemental constituents are used as starting materials. The preferably used solvent is water. The aqueous material obtained is then dried, the drying process preferably being effected by spray-drying the aqueous mixture with outlet temperatures of from 100 to 150xc2x0 C.
The multimetal oxide materials suitable according to the invention as active materials of the fixed-bed catalyst 2 for the second reaction stage, in particular those of the formula IV, can be used for the novel process both in powder form and shaped to give specific catalyst geometries, it being possible to effect the shaping before or after the final calcination. For example, unsupported catalysts can be prepared from the powder form of the active material or its uncalcined precursor material by compaction to give the desired catalyst geometry (for example by pelleting or extrusion), it being possible, if required, to add assistants, e.g. graphite or stearic acid as lubricants and/or molding assistants and reinforcing agents, such as microfibers of glass, asbestos, silicon carbide or potassium titanate. Suitable geometries for unsupported catalysts are, for example, solid cylinders or hollow cylinders having an external diameter and a length of from 2 to 10 mm. In the case of the hollow cylinders, a wall thickness of from 1 to 3 mm is expedient. The unsupported catalyst can of course also have spherical geometry, where the sphere diameter can be from 2 to 10 mm.
The shaping of the pulverulent active material or its pulverulent, still uncalcined precursor material can of course also be effected by application to premolded inert catalyst supports. The coating of the supports for the preparation of the coated catalysts is carried out as a rule in a suitable rotatable container, as disclosed, for example, in DE-A 2909671, EP-A 293859 or EP-A 714700.
For coating the supports, the powder material to be applied is expediently moistened and, after application, is dried again, for example by means of hot air. The coat thickness of the powder material applied to the support is expediently chosen to be in the range from 10 to 1000 xcexcm, preferably from 50 to 500 xcexcm, particularly preferably from 150 to 250 xcexcm.
Conventional porous or nonporous aluminas, silica, thorium dioxide, zirconium dioxide, silicon carbide or silicates, such as magnesium silicate or aluminum silicate, can be used as support materials. The supports may have a regular or irregular shape, those having a regular shape and substantial surface roughness, e.g. spheres or hollow cylinders, being preferred. The use of essentially nonporous, spherical steatite supports which have a rough surface and whose diameter is from 1 to 8 mm, preferably from 4 to 5 mm, is suitable. However, the use of cylinders whose length is from 2 to 10 mm and whose external diameter is from 4 to 10 mm as supports is also suitable. When rings suitable according to the invention are used as supports, the wall thickness is moreover usually from 1 to 4 mm. Annular supports preferably to be used according to the invention have a length of from 2 to 6 mm, an external diameter of from 4 to 8 mm and a wall thickness of from 1 to 2 mm. Rings measuring 7 mmxc3x973 mmxc3x974 mm (external diameterxc3x97lengthxc3x97internal diameter) are particularly suitable according to the invention as supports. The fineness of the catalytically active oxide materials to be applied to the surface of the support is of course adapted to the desired coat thickness (cf. EP-A 714 700).
Advantageous multimetal oxide active materials to be used according to the invention as fixed-bed catalysts 2 for the second reaction stage are furthermore materials of the formula VI,
[D]p[E]qxe2x80x83xe2x80x83(VI),
where
D is Mo12Vaxe2x80x3Z1bxe2x80x3Z2cxe2x80x3Z3dxe2x80x3Z4exe2x80x3Z5fxe2x80x3Z6gxe2x80x3Oxxe2x80x3,
E is Z712Cuhxe2x80x3Hixe2x80x3Oyxe2x80x3,
Z1 is W, Nb, Ta, Cr and/or Ce,
Z2 is Cu, Ni, Co, Fe, Mn and/or Zn,
Z3 is Sb and/or Bi,
Z4 is Li, Na, K, Rb, Cs and/or H
Z5 is Mg, Ca, Sr and/or Ba,
Z6 is Si, Al, Ti and/or Zr,
Z7 is Mo, W, V, Nb and/or Ta,
axe2x80x3 is from 1 to 8,
bxe2x80x3 is from 0.2 to 5,
cxe2x80x3 is from 0 to 23,
dxe2x80x3 is from 0 to 50,
exe2x80x3 is from 0 to 2,
fxe2x80x3 is from 0 to 5,
gxe2x80x3 is from 0 to 50,
hxe2x80x3 is from 4 to 30,
ixe2x80x3 is from 0 to 20 and
xxe2x80x3,yxe2x80x3 are numbers which are determined by the valency and frequency of the elements other than oxygen in VI
p and q are numbers which differ from 0 and whose ratio p/q is from 160:1 to 1:1,
which are obtainable by separately preforming a multimetal oxide material E
Z712Cuhxe2x80x3Hixe2x80x3Oyxe2x80x3xe2x80x83xe2x80x83(E),
in finely divided form (starting material 1) and then incorporating the preformed solid starting material 1 into an aqueous solution, an aqueous suspension or finely divided dry blend of sources of the elements Mo, V, Z1, Z2, Z3, Z4, Z5, Z6, which contains the abovementioned elements in the stoichiometry D
Mo12Vaxe2x80x3Z1bxe2x80x3Z2cxe2x80x3Z3dxe2x80x3Z4exe2x80x3Z5fxe2x80x3Z6gxe2x80x3xe2x80x83xe2x80x83(D),
(starting material 2), in the desired ratio p:q, drying any resulting aqueous mixture and calcining the resulting dry precursor material at from 250 to 600xc2x0 C. before or after its drying to give the desired catalyst geometry.
The multimetal oxide materials VI in which the incorporation of the preformed solid starting material 1 into an aqueous starting material 2 is effected at xe2x89xa770xc2x0 C. are preferred. A detailed description of the preparation of catalysts comprising multimetal oxide materials VI is contained, for example, in EP-A 668164, DE-A 19736105 and DE-A 19528646.
Regarding the shaping, the statements made in the case of the catalysts comprising multimetal oxide materials IV are applicable with respect to catalysts comprising multimetal oxide materials VI.
In the novel process, it is of course possible to use both fixed catalyst beds 1 and fixed catalyst beds 2 whose volume-specific activity in the direction of flow of the reaction gas starting mixture increases continuously, abruptly or stepwise (this can be effected, for example, as described in WO 98/24746, EP-A 450596, EP-A 792866 or JP-A 91/294239 or by dilution with inert material). According to the invention, it is also possible, in addition to nitrogen, steam and/or oxides of carbon, to use the inert diluent gases recommended in EP-A 293224 and in EP-B 257565 (e.g. only propane or only methane, etc.). The latter may, if required, also be used in combination with a volume-specific activity of the fixed catalyst beds which increases in the direction of flow of the reaction gas mixture.
According to the invention, the reaction zone B may be present spatially directly next to the reaction zone C or D, i.e. the fixed catalyst beds 1 and 2 may, according to the invention, be directly adjacent to one another. However, they can of course also be separated from one another by inert material beds and/or air.
Suitable separating inert materials are, for example, the inert materials separating reaction zones from one another in DE-C 2830765 and those in EP-A 911313. According to the invention, such an inert material bed is included with the fixed catalyst bed 2, i.e. is kept at the same temperature as the reaction zone C or D.
It is essential to the invention that both reaction stages, i.e. both the fixed catalyst beds 1 and 2 and the reaction zones A, B and C or A, B, D and E are present in a single tube-bundle reactor comprising a plurality of catalyst tubes.
In other words, the novel process must be realized in a tube-bundle reactor having a plurality of temperature zones, as disclosed by the example of two temperature zones in DE-C 2830765, DE-C 2513405, U.S. Pat. No. 3,147,084, DE-A 2201528, EP-A 383224 and DE-A 2903218.
In other words, the fixed catalyst beds 1 and 2 to be used according to the invention and an inert material bed which may separate them from one another are present spatially successively in the metal tubes of a tube-bundle reactor (in the case of a vertical orientation of the reaction tubes, expediently that fixed catalyst bed out of the two fixed catalyst beds whose time-on-screen is the shorter is present at the top) and heating media, as a rule salt melts, which are essentially spatially separated from one another, are passed around the metal tubes. The tube section over which the respective salt bath extends represents, according to the invention, a temperature zone, i.e. in the usual case according to the invention, a salt bath A flows around that section of the tubes (reaction zone A) in which the oxidative conversion of the propene (in a single pass) takes place until a conversion of from 40 to 80 mol % is reached, and a salt bath B flows around that section of the tubes (reaction zone B) in which the subsequent oxidative conversion of the propene (in a single pass) takes place until a conversion of at least 90 mol % is reached. In both abovementioned tube sections, the fixed catalyst bed 1 is present. A further salt bath C or two further salt baths D, E flow around the tube sections within which the fixed catalyst bed 2 and, if required, the inert material bed separating the two fixed catalyst beds from one another are present and in which the acrolein conversion takes place.
Usually, the beginning of the reaction zone B is behind the hot-spot maximum of the reaction zone A. In a corresponding manner, the beginning of the reaction zone E is usually behind the hot-spot maximum of the reaction zone D. As a rule, the reaction zone D extends up to a conversion of the acrolein, which emerges from the first reaction stage, of from 55 to 85 mol %, preferably from 65 to 80 mol %.
Each of the heating (salt) baths A, B, C or A, B, D, E can, according to the invention, be passed cocurrently or countercurrently through the space surrounding the reaction tubes, relative to the direction of flow of the reaction mixture flowing through the reaction tubes.
Expediently, cocurrent or countercurrent flow is used in all reaction zones. It is also advantageous to use cocurrent flow in reaction zone A and countercurrent flow in reaction zone B (or vice versa). The same applies to the reaction zones D, E.
In all abovementioned configurations within the respective reaction zone, it is of course possible also to superpose a transverse flow on the flow of the heating medium parallel to the reaction tubes, so that the individual reaction zone corresponds to a tube-bundle reactor as described in EP-A 700714 or in EP-A 700893 and a meandering flow of the heat exchange medium results overall in the longitudinal section through the catalyst tube bundle.
Expediently, the reaction gas starting mixture is preheated to the reaction temperature before being fed to the fixed catalyst bed 1.
In the abovementioned tube-bundle reactors, the catalyst tubes are usually made of ferritic steel and typically have a wall thickness of from 1 to 3 mm. Their internal diameter is as a rule from 20 to 30 mm, frequently from 21 to 26 mm. It is expedient in terms of application technology if the number of catalyst tubes housed in the tube-bundle container is at least 5000, preferably at least 10,000. Frequently, the number of catalyst tubes housed in the reaction container is from 15,000 to 30,000. Tube-bundle reactors having more than 40,000 catalyst tubes tend to be the exception. Inside the container, the catalyst tubes are usually homogeneously distributed, the distribution expediently being chosen so that the distance between the central inner axes of adjacent catalyst tubes (i.e. the catalyst tube spacing) is from 35 to 45 mm (cf. for example EP-B 468290).
Particularly suitable heat exchange media are fluid thermostating media. The use of melts of salts, such as potassium nitrate, potassium nitrite, sodium nitrite and/or sodium nitrate or of metals having a low melting point, such as sodium, mercury and alloys of various metals, is particularly advantageous.
In all abovementioned configurations of the flow in the multizone tube-bundle reactors, the flow rate within the required circulations of heat exchange media are as a rule chosen so that the temperature of the heat exchange medium increases by from 0 to 15xc2x0 C. from the point of entry into the reaction zone to the point of exit from the reaction zone (owing to the exothermic nature of the reaction), i.e. the abovementioned xcex94T may be, according to the invention, from 1 to 10xc2x0 C. or from 2 to 8xc2x0 C. or from 3 to 6xc2x0 C.
According to the invention, the temperature of the heat exchange medium on entering the reaction zone A is usually from 300 to 370xc2x0 C. According to the invention, the temperature of the heat exchange medium on entering the reaction zone B is usually on the one hand from 305 to 380xc2x0 C. and, on the other hand, simultaneously at least 5xc2x0 C. above the temperature of the heat exchange medium entering the reaction zone A.
Preferably, the temperature of the heat exchange medium on entering the reaction zone B is at least 10xc2x0 C. above the temperature of the heat exchange medium entering the reaction zone A. The difference between the temperatures on entering the reaction zones A and B may thus be, according to the invention, up to 10xc2x0 C., up to 20xc2x0 C., up to 25xc2x0 C., up to 30xc2x0 C., up to 40xc2x0 C., up to 45xc2x0 C. or up to 50xc2x0 C. Usually, however, the abovementioned temperature difference is not more than 50xc2x0 C. The higher the chosen propene loading of the fixed catalyst bed 1 in the novel process, the greater should be the difference between the temperature of the heat exchange medium on entering the reaction zone A and the temperature of the heat exchange medium on entering the reaction zone B.
According to the invention, the temperature of the heat exchange medium on entering the reaction zone B is advantageously from 305 to 375xc2x0 C., particularly advantageously from 305 to 370xc2x0 C.
In a corresponding manner, the temperature of the heat exchange medium on entering the reaction zone C is usually from 230 to 300xc2x0 C., preferably from 230 to 285xc2x0 C.
The temperature of the heat exchange medium on entering the reaction zone D is usually from 230 to 280xc2x0 C. (expediently from 245 to 275xc2x0 C.) and the temperature of the heat exchange medium on entering the reaction zone E is usually from 250 to 300xc2x0 C. (expediently from 265 to 295xc2x0 C.).
The temperature of the heat exchange medium on entering the reaction zone E is preferably at least 10xc2x0 C. or at least 20xc2x0 C. above the temperature of the heat exchange medium on entering the reaction zone D.
According to the invention, the chosen difference between the temperature of the heat exchange medium on entering the reaction zone E and that on entering the reaction zone D should be the greater the higher the chosen propene loading of the fixed catalyst bed 1. This difference is usually not more than 5xc2x0 C. However, it may be up to 15xc2x0 C., up to 25xc2x0 C., up to 30xc2x0 C., up to 35xc2x0 C. or up to 40xc2x0 C.
If, according to the invention, the fixed catalyst beds 1 and 2 are separated from one another by an inert material bed, the length of the inert material bed is usually xe2x89xa630%, frequently xe2x89xa620%, but as a rule xe2x89xa710% of the length of the fixed catalyst bed 2.
Such inert materials may be, for example, stainless steel coils, steatite rings or steatite beads. However, the inert materials may also be individual coils to be introduced into the reaction tubes and having a length of up to 50 cm (cf. EP-B 351167).
At this point, it should once again be pointed out that, for realizing the novel multizone procedure, it is possible also to apply in particular the principle which is described in German Published Application DAS 2,201,528 and which includes the possibility of transferring a portion of the hotter heat exchange medium of one reaction zone to a colder heat exchange medium of another reaction zone in order, if required, to heat up a cold reaction gas starting mixture or a cold recycled gas.
Furthermore, the tube bundle characteristic within an individual reaction zone may be designed as described in EP-A 382098.
It is noteworthy that, with a predetermined propene loading and predetermined conversion and selectivity, the novel procedure results in a substantially slower increase in the pressure drop in reaction zone C or in reaction zones D, E. This is due, inter alia, to the fact that the novel design of the first reaction stage very evidently reduces the volatilization of the MoO3, which finally actually permits the novel procedure to be carried out in a single tube-bundle reactor.
Another essential feature of the invention is that the novel process does not necessarily require the presence of an intermediate layer of inert material.
The novel process is particularly suitable for a continuous procedure.
The novel process gives not pure acrylic acid but a mixture from whose secondary components the acrylic acid can be isolated in a manner known per se (for example by rectification and/or crystallization). Unconverted acrolein and propene and inert diluent gas used and/or formed in the course of the reaction can be recycled to the gas-phase oxidation. In the novel two-stage gas-phase oxidation starting from propene, the recycling is expediently effected into the reaction zone A.
Otherwise, in this publication, conversion, selectivity and residence time are defined as follows, unless stated otherwise:                                           Conversion            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            starting                                                            material            ⁢                          xe2x80x83                        ⁢                          (              %              )                                            =                                                                      number                ⁢                                  xe2x80x83                                ⁢                of                ⁢                                  xe2x80x83                                ⁢                moles                ⁢                                  xe2x80x83                                ⁢                of                ⁢                                  xe2x80x83                                ⁢                starting                                                                                        material                ⁢                                  xe2x80x83                                ⁢                converted                                                                                                        number                ⁢                                  xe2x80x83                                ⁢                of                ⁢                                  xe2x80x83                                ⁢                moles                ⁢                                  xe2x80x83                                ⁢                of                ⁢                                  xe2x80x83                                ⁢                starting                                                                                        material                ⁢                                  xe2x80x83                                ⁢                used                                                        xc3x97      100                                                Selectivity            ⁢                          xe2x80x83                        ⁢            of                                                            product            ⁢                          xe2x80x83                        ⁢            formation                                =                                                                      number                ⁢                                  xe2x80x83                                ⁢                of                ⁢                                  xe2x80x83                                ⁢                moles                ⁢                                  xe2x80x83                                ⁢                of                ⁢                                  xe2x80x83                                ⁢                starting                                                                                        material                ⁢                                  xe2x80x83                                ⁢                converted                ⁢                                  xe2x80x83                                ⁢                into                ⁢                                  xe2x80x83                                ⁢                product                                                                                                        number                ⁢                                  xe2x80x83                                ⁢                of                ⁢                                  xe2x80x83                                ⁢                moles                ⁢                                  xe2x80x83                                ⁢                of                ⁢                                  xe2x80x83                                ⁢                starting                                                                                        material                ⁢                                  xe2x80x83                                ⁢                converted                                                        xc3x97      100                                                Residence            ⁢                          xe2x80x83                        ⁢            time                                                            (                          sec              .                        )                                =                                                                      empty                ⁢                                  xe2x80x83                                ⁢                reactor                ⁢                                  xe2x80x83                                ⁢                volume                ⁢                                  xe2x80x83                                ⁢                filled                                                                                        with                ⁢                                  xe2x80x83                                ⁢                catalyst                ⁢                                  xe2x80x83                                ⁢                                  (                  1                  )                                                                                                                        throughput                ⁢                                  xe2x80x83                                ⁢                of                ⁢                                  xe2x80x83                                ⁢                reaction                ⁢                                  xe2x80x83                                ⁢                gas                                                                                        starting                ⁢                                  xe2x80x83                                ⁢                mixture                ⁢                                  xe2x80x83                                ⁢                                  (                                      1                    ⁢                                                                  (                                                  S                          .                          T                          .                          P                          .                                                )                                            /                      h                                                        )                                                                        xc3x97      3600      